1. Field of the Invention
The present invention relates to an optical cable made of plastics (hereinafter referred to as "plastic optical cable").
2. Description of Prior Arts
Hitherto, optical fibers made from inorganic glass are known and widely used as optical signal-transmitting mediums. They are, however, poor in processability and weak against flexural stress. Due to these drawbacks of the glass optical fibers, attempts have been made to develop an optical fiber made of a plastic (hereinafter referred to as "plastic optical fiber"). As the plastic optical fiber, there is known a plastic fiber comprising a core made of a polymer having a comparatively large refractive index and good light transmission (e.g. methacrylic polymers and styrene base polymers) and a cladding made of a transparent polymer having a smaller refractive index than that of the core (e.g. fluorine-containing polymers) (hereinafter referred to as "bare fiber"). The conventional plastic optical fiber has still insufficient heat resistance so that its applications have been limited.
In view of the above circumstances, we have proposed an optical fiber with good heat resistance and light transmission which comprises a core made of a polymer comprising methyl methacrylate containing 3 to 40% by weight of a methacrylic ester the ester moiety of which has alicyclic hydrocarbon group of 8 or more carbon atoms and a cladding made of a transparent resin or a fluororubber having a refractive index at least 3% smaller than that of the core polymer (cf. U.S. Ser. No. 504,861 filed on June 16, 1983 and European Patent Application No. 83 105 869.8 filed on June 15, 1983 corresponding to Eur. Pat. No. 0 097325).
For the purpose of applying the bare fiber of the optical fiber comprising the core and the cladding to an optical trnasmission system, it is necessary to fabricate an optical cable from the bare fiber by coating it with a resin, namely by forming a protective layer thereon. As an example of the cable fabrication techniques, Japanese Patent Kokai Publication (unexamined) No. 226302/1984 proposes coating of the bare fiber with a thermoplastic resin such as polyvinyl chloride to produce an optical cable having improved heat resistance and mechanical properties.
However, since the optical cable coated with the thermoplastic resin has a large degree of thermal shrinkage, it suffers from a decrease of optical transmission due to thermal shrinkage when it is installed in or near a hot place such as an engine room of an automobile or a vessel. Thus, it lacks reliability in practical use and is not satisfactory as a practically usable optical cable.